Power factor corrected UPS with improved connection of battery to neutral and methods of operation thereof

ABSTRACT

An uninterrupted power supply (UPS) device with uninterrupted neutral from input to output utilizes the same converter for converting rectified AC power and battery power to positive and negative high voltage (HV) rails. A simple circuit is utilized for connecting the battery to the conversion components of the PFC circuit without adverse affect on the performance of the PFC circuit, and while holding the battery substantially connected to neutral. In a first embodiment, the circuit comprises a simple combination of four diodes and a pair of high pass capacitors arranged so that in both power line and battery supply modes the battery is balanced around neutral. In a second, preferred embodiment, one terminal of the battery is connected directly to neutral.

RELATED APPLICATIONS

This a continuation of prior application Ser. No. 09/812,993 filed Mar.20, 2001 now U.S. Pat. No. 6,400,586, which is a continuation of priorapplication Ser. No. 09/563,462 filed May 2, 2000 now U.S. Pat. No.6,262,899, which is continuation of application Ser. No. 08/038,469filed Mar. 29, 1993, now U.S. Pat. No. 6,069,412, the disclosures ofwhich are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to uninterrupted power supply (UPS) apparatusand, more particularly, to a power factor corrected UPS maintainingintegrity of the connection from power line neutral to an output loadterminal.

2. Description of the Prior Art

UPS systems are now widely used to provide a secure supply of power tocritical loads such as computers, so that if the line voltage varies oris interrupted, power to the load is maintained at an adequate level andis not lost. The UPS conventionally comprises a rectifier circuit forproviding a DC voltage from the AC power lines; an inverter forinverting the DC voltage back to an AC voltage corresponding to theinput, for delivery to the load; and a battery and a connection circuitfor connecting battery power to the input of the DC to AC inverter, sothat when reliable AC power is lost the delivery of AC power to the loadis substantially unaffected. In such an UPS, it is highly desirable tomaintain an uninterrupted neutral from the commercial AC utility powerto each component circuit and to the load, e.g., in order to eliminateshock hazards. Because of the inherent nature and mode of operation oftypical UPS systems, conventional UPS designs did not maintain theintegrity of the neutral through the processing circuitry, requiringsome type of isolation means such as isolation transformer tore-establish the neutral at the load. U.S. Pat. No. 4,935,861, assignedto the assignee of this invention, provides an UPS wherein theelectrical continuity of an electrical conductor is maintained from oneterminal of the AC utility through to one of the load terminals, withoutany isolation means being required.

The problem with maintaining integrity of the neutral is furthercomplicated in a UPS having a power factor correction circuit. The taskof connecting the battery to neutral is simple in a power supply unitwithout a PFC circuit, such as shown in U.S. Pat. No. 4,823,247. But asis well known, there are important reasons for incorporating powerfactor correction (PFC) into an UPS. And, the incorporation of such aPFC circuit imposes additional difficulties upon the goal of maintainingintegrity of a neutral connection from the power line to the load. Adesign for achieving an uninterrupted power supply system having a PFCcircuit is disclosed in U.S. Pat. No. 4,980,812, also assigned to theassignee of this invention.

It is recognized that maintaining the integrity of the neutral in an UPSoffers advantages of lower cost, due to lack of need for isolationmeans, and higher reliability. Because of the design criterion of anundisturbed neutral, an UPS with a PFC circuit has heretofore requiredthree converters. As seen in FIG. 1, such a prior art apparatus containsa converter as part of the power factor correction circuit, the outputof which provides DC on a positive high voltage (HV) rail andindependent negative HV rail respectively relative to the neutral line.The DC-AC inverter is necessarily a second converter, and, a thirdconverter circuit has been necessary to connect the DC from the batteryto the HV rails. Prior art attempts to combine the battery converterwith the PFC converter have always resulted in either an isolated UPS,wherein the neutral is not maintained, or some circuit arrangement forconnecting the DC output of the battery into an AC voltage which couldbe utilized by the AC to DC converter portion of the PFC circuit. Forsafety reasons, it is desirable to effectively connect the battery tothe neutral, which leaves an unfulfilled need for an efficient andreliable manner of translating the battery output to the HV rails. Thedesign solution of having a third converter of some different kind, orthe option of using an isolation transformer, both have obviousdisadvantages. The problem is thus how to provide that the convertedoutput from the PFC circuit, as well as the battery output, can beindependently loaded and still balanced around neutral to the plus andminus HV rails without using a separate converter of some sort for each.Stated differently, the problem for which a solution has not heretoforebeen known is how to connect the battery to the HV rails utilizing thePFC converter, while-effectively maintaining a connection from thebattery to neutral.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a power factor correctedUPS which maintains neutral integrity from the input of the UPS to anoutput terminal to which the load is connected, the UPS device having asimple and efficient circuit for connecting the battery to the converterof the PFC circuit, whereby whenever the battery provides output powerdue to deterioration of the utility line voltage, battery voltage isconverted through the PFC converter and delivered to the high voltagerails. The UPS achieving this object provides an uninterrupted neutralfrom its input connection to the AC power line through to an outputterminal for connection to the load, balances the battery aroundneutral, and achieves supply of the battery power independently to thehigh voltage rails without the need of an independent battery to HV railconverter, or the need for any isolation means.

In a first embodiment, a four diode-two capacitor circuit is used toconnect the battery to the PFC converter. During normal operation whenthe UPS is drawing power from the utility line, the battery is balancedaround neutral and is maintained no more than one forward diode dropaway from neutral. By using a battery with a voltage less than one-halfof the peak of the incoming AC voltage, the PFC circuit is substantiallyunaffected so that power factors greater than 0.9 can be achieved.During loss of AC input, when the UPS runs on battery, switchingelements of the PFC converter are independently turned on and off,enabling conversion of the battery voltage through the PFC convertercircuitry to the HV lines. In a second, preferred embodiment, oneterminal of the battery is connected directly to neutral, and the otherterminal is connected through a normally open switch and a diode to theconverting circuit. The switch is closed when low AC power line voltageis sensed. Both embodiments thus enable elimination of a separateconverter for the battery while preserving the advantages of prior artpower factor corrected UPS devices maintaining integrity of the neutralconnection from input to load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram showing the primary components of aprior art power factor corrected UPS.

FIG. 2 is a simplified circuit diagram of a power factor corrected UPSwith neutral integrity, and illustrating the problem of connecting thebattery to the HV rails without the aid of a converter dedicated to thebattery.

FIG. 3 is a circuit diagram showing a first embodiment of the improvedconnection circuit of this invention, whereby the battery is connectedto the converter of the PFC circuit while maintaining the batterybalanced around neutral.

FIGS. 4A and 4B are circuit diagrams illustrating a cycle of operationwhen the UPS of FIG. 3 is drawing power from the AC input, and the lineor energized AC input terminal is positive relative to the neutralterminal.

FIGS. 5A and 5B are circuit diagrams illustrating a cycle of operationwhen the UPS of FIG. 3 is drawing power from the AC input, and the lineor energized AC input terminal is negative relative to the neutralterminal.

FIGS. 6A and 6B illustrate operation of the improved UPS circuit of FIG.3 during a condition of unacceptable AC input and UPS battery operation.

FIG. 7A is a circuit diagram of a preferred embodiment of the invention,wherein one terminal of the battery is connected directly to neutral.

FIGS. 7B and 7C are circuit diagrams illustrating a cycle ofbattery-driven operation for the circuit of FIG. 7A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, there is shown a circuit diagram of a typicalpower factor corrected UPS with an uninterrupted neutral from input tooutput. The AC input is connected to the UPS at two input terminals, oneof which is marked “line” and the other of which is marked “neutral.”The neutral line is connected by an uninterrupted conductor to one oftwo output terminals, across which AC output power is delivered. The ACinput signal is connected across a first capacitor C1. The line terminalis connected to rectifier diodes D1 and D2. D1 is in series withinductor L1, the other side of L1 being connected through switchingtransistor Q1 to neutral. D2 is connected in series with inductor L2,the other side of L2 being connected through switching transistor Q2 toneutral. The input terminals 31, 32 are driven by switch control means33 such as illustrated in FIG. 1 of U.S. Pat. No. 4,980,812,incorporated herein by reference. Transistors Q1 and Q2 of FIG. 2correspond to transistors 86 and 88 seen in FIG. 1 of the referencedpatent. Transistors Q1 and Q2 are driven in such a manner as to achievea power factor close to 1.0, and to maintain needed voltage across C2and C3. Inductor L1 is also connected through diode D3 and capacitor C2to neutral; and inductor L2 is connected through diode D4 and capacitorC3 to neutral. When Q1 is turned off after it has been conducting,current is passed through L1 and D3 to charge capacitor C2, maintainingpositive voltage on the +HV rail 35. Likewise, when Q2 is turned offafter having been turned on during a negative swing of the line voltage,current from inductor L2 passes through diode D4 and charges capacitorC3, maintaining negative voltage on high V rail 36.

Still referring to FIG. 2, HV rails 35 and 36 have connectedtherebetween transistor switches Q3 and Q4 in series, which are drivenat input terminals 38 and 39 by a reference signal in a well knownmanner, so as to alternately switch on during respective half cycles ofpositive and negative going voltage. Diode D5 is placed acrosstransistor Q3, and diode D6 is placed across transistor Q4. The switchedvoltage appearing at the node between transistors Q3 and Q4 is connectedto filtering inductor L3, and the AC output which appears acrosscapacitor C4 drives the load 40 connected between line out and neutral.

Battery 30 is shown in FIG. 2, having its negative terminal connected toneutral, but its positive terminal unconnected. The longstanding problemin the art, which this invention meets, is how to connect the battery insuch a way as to enable generation of the plus and minus HV rails fromsuch battery at the time of AC input line failure. What is needed is asimple but reliable circuit which can utilize the inductor and switchingcomponents of the PFC circuit, i.e., inductors L1 and L2, andtransistors Q1 and Q2.

Referring now to FIG. 3, there is shown an improved circuit whichconnects the battery to converter elements of the power factorcorrection circuit of FIG. 2. In addition to the circuit componentsillustrated in FIG. 2, there is illustrated a battery 30 which is tiedat its plus terminal to neutral through diode D9, and at its minusterminal to neutral through diode D10. Bypass capacitors C5 and C6bridge diodes D9 and D10 respectively, and are chosen to have a largecapacitance with respect to the switching frequency of switches Q1 andQ2, which is determined by control circuit 33. The positive terminal ofthe battery is also connected through D7 to a node between D1 and L1,and the negative terminal of the battery is connected through diode D8to a node between D2 and L2. Instead of connecting Q1 and Q2 to neutralas in FIG. 2, the emitter of Q1 is connected to the negative terminal ofthe battery, while the collector of Q2 is connected to the positiveterminal of the battery. Thus, in terms of extra circuit components, theimproved circuit comprises the simple addition of four diodes and twohigh frequency bypass capacitors. During normal operation the battery isbalanced around neutral, and never gets more than a forward biased diodedrop away from neutral, e.g., about one-half to three-fourths volts. Byutilizing a battery that has a voltage less than one-half the peak ofthe incoming AC voltage, the power factor correction circuit operatesover a sufficiently long portion of each cycle to achieve a power factorgreater than 0.9.

Referring now to FIGS. 4A and 4B, there are illustrated circuit diagramsshowing the equivalent circuit operation under conditions where there isa good input on the AC line, and the input voltage is positive andgreater than battery voltage. In FIG. 4A, Q1 is illustrated in an on orclosed switch position, and in FIG. 4B is illustrated in an off, or openswitch position. Note that Q1 is turned on only when the voltage peak isgreater than the battery voltage, such that D7 is reversed biased. Inthis condition, as illustrated in referenced U.S. Pat. No. 4,980,812,capacitor C2 is shunted by Q1 and current builds up in inductor L1. WhenQ1 opens, as shown in FIG. 4B, L1 acts as a current generator and pumpscurrent into capacitor C2, building up the DC voltage thereacross. FIGS.5A and 5B show the equivalent circuit diagram when the line terminal isnegative and the voltage exceeds the battery voltage. In a similarfashion, when Q2 is closed and thus shunts C3, current builds up throughL2. When Q2 is opened, current is pumped from L2 into capacitor C3,thereby generating a negative voltage across C3 with respect to neutral.These respective operations generate the positive and negative HV railsindicated in FIG. 3, in a manner that is substantially unchanged withrespect to the embodiment of U.S. Pat. No. 4,980,812. During thistypical cycle of operation, forward biased diode D10 connects currentthrough Q1 while it is closed, and forward biased diode D9 is in serieswith switch Q2 when it is closed, with the result that the improvedcircuit has no appreciable impact on the operation of the PFCconversion. During the positive line voltage swing, the negativeterminal of the battery is tied to neutral through D10; during thenegative line voltage swing, the positive terminal of the battery istied to neutral through D9.

Referring now to FIGS. 6A and 6B, there are illustrated the effectivecircuit diagrams for the UPS circuit of this invention during loss of ACinput, i.e., at any time when UPS load is being supplied by the battery.During this time, the improved switching circuit acts to connect thebattery to alternately charge C2 and C3 so as to maintain the same plusand minus high voltage rails. During such battery back up operation,switches Q1 and Q2 are turned on and off independently, by switchcontrol 33.

When the AC source voltage drops to an unacceptable level, switchcontrol 33 operates to drive Q1 and Q2 through on-off cycles, at a dutycycle as required to provide a regulated output. Note that each of Q1and Q2 can be switched independently, as may be required for anunbalanced load (not shown unbalanced). Q2 is held off (open) while C2is charged, and Q1 is held off while C3 is charged.

During the period of time that Q2 is held off, Q1 is first switched onand then switched off. FIG. 6A shows Q2 off and Q1 switched on. Underthese circumstances, current flows from the battery through diode D7,inductor L1, and back through switch Q1 to the negative terminal of thebattery, building up current flow in inductor L1. At the same time,remaining current through L2 is discharged through diode D8, diode D10,capacitor C3 and diode D4. When Q1 is turned off (FIG. 6B), the build upof current is passed through diode D3 into capacitor C2, charging itpositively with respect to neutral. The current through C2 returnsthrough diode D9. At the same time, current from battery 30 goes aroundthe outer loop of the circuit shown, i.e., through D7, L1, D3, C2, C3,D4, L2 and D8. Following this, the sequence is reversed such that Q1 isturned off, and Q2 is alternatingly turned on and off, resulting in thereverse operation which builds up the negative voltage across capacitorC3. During the battery supply of the output voltage, if capacitor C2 andC3 are loaded in a balanced manner, and if C5 and C6 have largecapacitance for the switching frequency, then the voltage across each ofcapacitors C5 and C6 is held substantially constant and has a value ofapproximately one-half the voltage of the battery. To the extent that C2and C3 loading becomes unbalanced, the ratio of the voltages across C5and C6 likewise is unbalanced.

Referring now to FIG. 7A, there is shown a preferred circuit. In thisembodiment, battery 30 has one terminal (illustrated as the negativeterminal) connected to neutral. The other terminal is connected throughswitch S1 to D7. Switch S1 is normally open, but is closed by control 33whenever low line voltage is detected, in a conventional manner.Compared to FIG. 3, diode D10 and capacitor C6 are eliminated, andswitch S1 is added. FIGS. 7B and 7C illustrate the circuit action whenthe load is battery driven. In FIG. 7B, each of switches Q1 and Q2 areclosed, such that current flows from battery 30 to each inductor L1, L2.In FIG. 7C, Q1 and Q2 are each switched open, so that current flows fromL1 to C2, and from L2 to C3. In this embodiment as well, switch control30 can drive Q1 and Q2 independently when the UPS is in thebattery-driving mode due to low source AC voltage.

Both the preferred embodiment of FIG. 7A and the embodiment of FIG. 3illustrate a DC to AC converter (utilizing transistors Q3, Q4), forproviding uninterrupted AC output. However, the invention also appliesto a supply for providing a DC output, such that no DC to AC inverter isutilized. Thus, in general, the invention comprises an output circuitbetween the HV rails and the output terminals.

There is thus illustrated a very simple, inexpensive and reliablecircuit which achieves the object of connecting the battery to an UPShaving an uninterrupted neutral from input to output, the batteryconnection being made in such a way as to utilize the PFC circuit forconversion of the battery voltage during times when the battery issupplying output load. At the same time, the circuit ties one terminalof the battery to neutral, or holds the battery balanced around neutral,and does not adversely affect performance of the PFC circuit. Theinvention thus achieves the object of allowing the battery to beconnected to neutral at all times, while utilizing the PFC circuit toconvert the battery output to the HV lines at the time of AC powersource failure.

What is claimed:
 1. An uninterruptible power supply (UPS) apparatus,comprising: a power factor correcting converter circuit, configured toconnect to an AC power source and to a DC power source, that generates aDC output voltage from the AC power source in an AC powered mode andthat generates the DC output voltage from the DC power source in a DCpowered mode.
 2. An uninterruptible power supply (UPS) apparatus,comprising: a power factor correcting converter circuit, configured toconnect to an AC power source and to a DC power source, that generates aDC voltage from respective ones of the AC power source and the DC powersource in respective AC powered and DC powered modes; and an outputcircuit, coupled to the power factor correcting converter circuit, thatgenerates an AC output from the DC voltage such that the AC outputshares a common neutral with the AC power source.
 3. An uninterruptiblepower supply (UPS) apparatus, comprising: a power factor correctingconverter circuit having a neutral configured to connect to an AC powersource and to a DC power source, the power factor correcting convertercircuit operative to generate a DC voltage with respect to the neutralfrom respective ones of the AC power source and the DC power source inrespective AC powered and DC powered modes; and an output circuitcoupled to the power factor correcting converter circuit and operativeto generate from the DC voltage an AC output with respect to theneutral.
 4. An uninterruptible power supply (UPS) apparatus, comprising:a boost converter circuit, configured to connect to an AC power sourceand to a DC power source, that generates a DC voltage from respectiveones of the AC power source and the DC power source in respective ACpowered and DC powered modes using at least one common inductor; and anoutput circuit, coupled to the converter circuit, that generates an ACoutput from the DC voltage such that the AC output shares a commonneutral with the AC power source.
 5. An uninterruptible power supply(UPS) apparatus, comprising: a power factor correcting convertercircuit, configured to connect to an AC power source and to a DC powersource, that generates a DC voltage from respective ones of the AC powersource and the DC power source in respective AC powered and DC poweredmodes using at least one common inductor; and an output circuit, coupledto the power factor correcting converter circuit, that generates an ACoutput from the DC voltage.
 6. An uninterruptible power supply (UPS)apparatus, comprising: an inductor; a DC voltage rail; a switch circuitcoupled to the inductor and to the DC voltage rail and operative totransfer power to via the inductor from respective ones of an AC powersource and a DC power source in respective AC and DC powered modes togenerate a DC voltage at the DC voltage rail; and an output circuit,coupled to the DC voltage rail, that generates an AC output from the DCvoltage, wherein the AC output shares a common neutral with the AC powersource.
 7. A method of generating an AC output, comprising; connectingrespective ones of an AC power source and a DC power source to an inputof a power factor correcting converter circuit in respective AC and DCpowered modes to generate a DC voltage with respect to a neutral of theAC power source from respective ones of the AC power source and the DCpower source; and generating an AC output with respect to the neutralfrom the DC voltage.
 8. A method of producing an AC output, comprising:transferring power to a DC voltage rail from respective ones of an ACpower source and a DC power source via a common inductor in AC and DCpowered modes to generate a DC voltage at the DC voltage rail; andgenerating an AC output from the DC voltage such that the AC outputshares a common neutral with the AC power source.